In the field of the fabrication of composite structures, it is often useful to be able to assemble and/or separate films or layers, such as, for example, semiconducting or insulating layers. Such separations are, in particular, necessary in order to transfer a layer from an initial substrate to a final substrate. These transfers are carried out, for example, during the implementation of three-dimensional component technology, which involves the formation of electronic, photovoltaic and/or optoelectronic components on both faces (“front face” and “rear face”) of the same layer (3D integration). Layer transfers are also carried out in order to transfer circuits during the fabrication of rear-face illuminated imagers. The transfer of layers is also useful for changing the substrate on which one or more layers are formed, so that the new substrate meets requirements in terms particularly of cost, physical characteristics (cell size, thermal stability, . . . ), etc.
A method of thin-film transfer is described, for example, in patent document EP 0 858 110. This method provides, in particular, the separation of a film with the aid of a technique of separation by exfoliation, this technique requiring, in particular, irradiation of a composite structure through a transparent substrate.
An exemplary embodiment of a method for fabricating a composite structure (steps S1 and S2) and of a method of separation by exfoliation (steps S3 and S4) will now be described with reference to FIG. 1.
First, a so-called separation layer 10 (or optical absorption layer) is assembled by bonding on one of the faces of a support substrate 5 (step S1). As indicated in more detail below, the support substrate 5 is at least partially transparent at a predetermined wavelength.
A layer 15 (also referred to as “layer to be separated”) is subsequently assembled by bonding on the face of the separation layer 10 on the opposite side from the one in contact with the support substrate 5, so as to obtain a composite structure 25 (step S2).
It will be noted that the assembly of the layers 5, 10 and 15 during steps S1 and S2 may be carried out by means of any suitable assembly technique, such as, for example, a technique of molecular adhesion bonding or involving an intermediate bonding layer.
Furthermore, the layers 10 and 15 are not necessarily assembled by bonding in order to form the composite structure 25. As a variant, at least one of the layers 10 and 15 may be formed by means of a suitable deposition technique. The separation layer 10 may, for example, be formed by PECVD (“plasma-enhanced chemical vapor deposition”) or LPCVD (“low-pressure CVD”) deposition.
Once the composite structure 25 has been formed, separation of the separation layer 10 may be carried out by exfoliation. This method makes it possible to detach the layer 15 from the support substrate 5.
To this end, the separation layer 10 is irradiated by means of electromagnetic radiation 20 through the support substrate 5 (step S3). The radiation 20 is at a wavelength for which the support substrate 5 is at least partially transparent. Here, “partially transparent” is intended to mean a substrate whose transmittance at the wavelength in question is at least 10%, and preferably greater than or equal to 50%. As indicated below, the required level of transparency will vary according to the amount of energy of the electromagnetic beam 20, which is received by the separation layer 10.
During this irradiation step S3, the separation layer 10 absorbs the incident light passing through the interface 8 between the support substrate 5 and the separation layer 10. This irradiation leads to a reduction or elimination of the adhesion forces between the atoms or molecules in the material of the separation layer 10. This is because, under the action of the radiation 20, the material constituting the separation layer 10 is subjected to photochemical and/or thermal excitation, which leads to the breaking of a chain of atoms or molecules. These breaks thus cause separation of the separation layer 10 by exfoliation, either in the actual thickness of the layer 10 (so-called “internal” exfoliation) or at the interface 8 between the layer 10 and the support substrate 5 or at the interface 12 between the separation layer 10 and the layer 15 to be separated (“interfacial” exfoliation). This exfoliation phenomenon may also involve one or more gases released by the material of the separation layer 10 under the action of the radiation 20.
It should be noted that the separation induced by the radiation 20 does not necessarily lead to detachment or actual separation in the separation layer 10 (or at one of the interfaces 8 and 12), but may simply lead to weakening of the material of the separation layer 10. In the latter case, the application of additional energy (for example, in the form of mechanical forces) is necessary in order to obtain the actual detachment between the support substrate 5 and the layer 15 (if such detachment is actually desired).
Once the substrate 5 and the layer 15 have been fully separated (step S4), the support substrate 5 may be recycled with a view to forming a new composite structure.
Currently, the composite structures produced according to the layout of FIG. 1 generally have one of the following compositions:                GaN/Al2O3, which corresponds to a separation layer 10 consisting of GaN and a support substrate 5 consisting of sapphire;        Si3N4/Al2O3, which corresponds to a separation layer 10 consisting of Si3N4 and a support substrate 5 consisting of sapphire.        
With regard to these compositions, the results in terms of quality of separation by exfoliation are, in general, satisfactory. When layers of GaN deposited on a sapphire substrate are separated, for example, the application of the radiation 20 (at a wavelength of typically between 190 nm and 250 nm) takes place under good conditions and the separation is obtained without any particular difficulty.
The applicants have, however, observed that the results can be significantly degraded when this separation method is applied to other compositions of the composite structure 25. For example, the separation by exfoliation is much more difficult for a composite structure 25 of the SiO2/Si type (i.e., silicon dioxide on silicon). The applicants have observed large variations in terms of quality of separation by exfoliation as a function of the batches studied and, in general, less uniform separations requiring more exposure to radiation.
There is, therefore a current need for composite structures better suited to the method of separation by exfoliation, irrespective of the composition of the composite structures in question. There is, in particular, a need for composite structures of unconventional composition adapted to be separated by exfoliation under good conditions.